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Guidebook 22. Field Trip Guidebook Landforms, Stratigraphy, and Lithologic Characteristics of Glacial Deposits in Central Minnesota
(Minnesota Geological Survey, 2006) Knaeble, A.R.
This guidebook was prepared for the 50th Midwest Friends of the Pleistocene Field Conference, sponsored by the Minnesota Geological Survey and held at St. John's University on June 4-6, 2004. The purpose of this guidebook is to provide conference participants with an up-to-date general summary of the glacial geology of central Minnesota and a comprehensive reference list of previous research completed in the area. There are a number of reasons why we were motivated to host the Friends of the Pleistocene in central Minnesota. First, over the last 10 years the Minnesota Geological Survey has completed numerous mapping projects in the central Minnesota area (Steams, Pope, and Crow Wing County atlases; the Otter Tail regional hydrogeological assessment; and the USGS Statemap St. Cloud, Baxter, Brainerd, and Gull Lake quadrangle maps), and is in the process of completing mapping projects in Todd, Traverse, and Grant Counties. The large volume of data (samples and descriptions from outcrops and drill holes) collected from these projects has allowed us to evaluate the work done by previous researchers and contribute new insights and interpretations. Second, in 1954 Herb Wright, AI Schneider, and Harold Ameman led the 5th Midwest Friends of the Pleistocene field trip in central Minnesota. We will revisit the area, on this 50th anniversary of that trip, to examine how interpretations have changed and evolved. The guidebook will use a simple, direct approach to summarize the region's glacial geology, similar to that used in 1954. It is our intention to pay tribute to the accomplishments of Herb and AI during the field trip and banquet. We will also acknowledge other researchers who, over the last 50 years, have made contributions to the glacial geology of central Minnesota. Third, we would like to discuss how mapping techniques and technology have changed in the last 50 years. We will examine which techniques have been most effective in understanding the complex stratigraphy of central Minnesota. Finally, field exposures were selected to stimulate interest and discussion about the following glacial topics: erosion, transport, and deposition of source-area materials; processes involved in drumlin formation; the relationship between ice dynamics and glacial landforms; processes important in the formation of the St. Croix moraine; and the challenges of interpreting thick, complex drift stratigraphy. These stops highlight geomorphic features, stratigraphic relationships, and specific unit characteristics (lithology, color, etc.) in an attempt to provide an overview of the glacial geology of this region.
Guidebook 21. Field Trip Guidebook for Selected Geology in Minnesota and Wisconsin
(Minnesota Geological Survey, 2005) Robinson, Lori, Editor
CONTENTS Field Trip 1. THE GEOLOGY OF THE MISSISSIPPI RIVER VALLEY-TWIN CITIES REGION: USING AN URBAN RIVER FOR INQUIRY-BASED EARTH SCIENCE EDUCATION Karen Campbell and Kent Kirkby Field Trip 2. CLASSIC PRECAMBRIAN GEOLOGY OF NORTHEAST MINNESOTA Mark Jirsa and James Miller Field Trip 3. DEPOSITS AND LANDFORMS IN THE REGION GLACIATED BY THE ST. LOUIS SUBLOBE Alan Knaeble, Gary Meyer, Lisa Marlow, Phil Larson, and Howard Mooers Field Trip 4. GRANITES OF THE EAST-CENTRAL MINNESOTA BATHOLITH Terry Boerboom, Daniel Holm, and Randy Van Schmus Field Trip 5. SINKHOLE ANATOMY 101 Calvin Alexander, Howard Hobbs, and Robert Tipping Field Trip 6. LATE ORDOVICIAN STRATIGRAPHY AND PALEONTOLOGY OF THE TWIN CITIES BASIN Robert Sloan, Michael Middleton, and Gerald Webers Field Trip 7. LATE ORDOVICIAN LITHOSTRATIGRAPHY AND BIOSTRATIGRAPHY OF THE SOUTHERN MARGIN OF THE TWIN CITIES BASINRobert Sloan, Michael Middleton, and Gerald Webers Field Trip S. GEOLOGY OF THE PRE-CRETACEOUS WEATHERING PROFILE AND CRETACEOUS STRATA OF THE MINNESOTA AND COTTONWOOD RIVER VALLEYS Larry Zanko, John Heine, and Dale Setterholm Field Trip 9. ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT: STRATIGRAPHY, STRUCTURE, AND MINERALIZATION Dean Peterson, Mark Jirsa, and George Hudak Field Trip 10. THE WESTERN MARGIN OF THE KEWEENAWAN MIDCONTINENT RIFT SYSTEM: GEOLOGIC HIGHLIGHTS OF ARCHEAN, PALEOPROTEROZOIC, MESO PROTEROZOIC, AND PALEOZOIC BEDROCK IN EASTERN MINNESOTA AND NORTHWESTERN WISCONSIN Terry Boerboom, Daniel Holm, Laurel Woodruff, and Karl Wirth Field Trip 11. GEOLOGY AND SEDIMENTOLOGY OF THE PALEOPROTEROZOIC ANIMIKIE GROUP: POKEGAMA FORMATION, BIWABIK IRON FORMATION, AND VIRGINIA FORMATION OF THE EASTERN MESABI IRON RANGE, AND THE THOMSON FORMATION NEAR DULUTH, NORTHEASTERN MINNESOTA Richard Ojakangas, Mark Severson, Peter Jongewaard, John Arola, Joel Evers, Doug Halvorson, G.B. Morey, and T.B. Holst Field Trip 12. PRE-WISCONSINAN AND WISCONSINAN GLACIAL. STRATICRAPHY, HISTORY, AND LANDSCAPE EVOLUTION, WESTERN WISCONSIN Kent Syverson, Robert Baker, Steven Kostka, and Mark Johnson
Guidebook 20. Field Trip Guidebook for the Geology and Ore Deposits of the Midcontinent Rift in the Lake Superior Region
(Minnesota Geological Survey, 1995) Miller, James D., Jr.
This guidebook has been prepared for the field conference and symposium on "The Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System" that will be held in Duluth, Minnesota in August of 1995 as part of International Geological Correlation Program (IGCP) Project 336 ("Petrology and Metallogeny of Mafic and Ultramafic Magmatism"). The four-year-Iong IGCP project was begun in 1992 to investigate the petrology and metallogeny of various intracontinental mafic igneous provinces with an overall goal of establishing geological criteria for targeting mineral deposits in such environments. The principal goal of the 1995 meeting is to showcase to the international scientific and exploration communities the significant advances in our understanding of the structure, mineralization, and magmatic history of the Midcontinent rift. Toward this end, three field trips have been organized around the three-day symposium to be held August 25-27, 1995 in Duluth. The field trips are designed to provide a broad overview of the geologic framework of the Midcontinent rift, where it is best exposed in the western Lake Superior region. Field trip I (August 19 -24) looks at the physical volcanology of the flood basalts exposed on Isle Royale. Field trip II (August 22-24) highlights the stratigraphy, structure and mineralization of the volcanic and sedimentary rocks forming the southern limb of the Midcontinent rift in Wisconsin and Upper Michigan. The post-symposium field trip ill (August 28-September 1) focusses on the geology and ore deposits associated with the intrusive rocks of the Midcontinent rift exposed along the north shore of Lake Superior in Minnesota and Ontario.
Guidebook 19. Field Trip Guidebook Kaolinitic Clays of the Minnesota River Valley and Southwestern Minnesota
(Minnesota Geological Survey, 1992) Setterholm, Dale R.; Heine, John J.
A thick mantle of chemical weathering products covers crystalline bedrock of Archean and Proterozoic age in much of central and western Minnesota. The mantle has a vertical profile typically composed of progressively more weathered material upward from fresh bedrock to an erosion surface now covered either by strata of Late Cretaceous age or by glacial and postglacial deposits of Quaternary age. In a pioneering study of regional relationships, Parham (1970) showed that most upland areas underlain by crystalline bedrock in western Minnesota have weathering profiles averaging 30 m in thickness, though profiles as great as 60 m thick may be present locally. Weathered materials are considerably thinner on steep side slopes and absent in many places along valleys cut into the bedrock surface. Nonetheless, their widespread occurrence has created considerable commercial interest. Much of our knowledge of the processes responsible for the origin of the weathering profile in southwestern Minnesota was established by S.S. Goldich in a 1936 University of Minnesota doctoral dissertation entitled "A study in rock weathering" (Goldich, 1938). Six samples of saprolite from the Redwood Falls-Morton vicinity in the Minnesota River Valley were compared chemically and mineralogically with fresh Archean gneiss, their probable parent material. Using these data, together with data from elsewhere, Goldich established an alteration sequence in which calcium-rich plagioclase was transformed to clay, followed by the dissolution of the potassium-rich feldspar and other mafic minerals. These observations are embodied in the so-called Goldich stability series, which in one form or another is taught to every beginning student in geology.
Guidebook 18. Field Trip Guidebook Glacial Geology of the Laurentian Divide Area, St. Louis and Lake Counties, Minnesota
(Minnesota Geological Survey, 1992) Lehr, J.D.; Hobbs, H.C.
CONTENTS GLACIAL GEOLOGY OF THE LAURENTIAN DIVIDE AREA, ST. LOUIS AND LAKE COUNTIES, MINNESOTA, J.D. Lehr and Howard C. Hobbs AN OVERVIEW OF THE VEGETATIONAL HISTORY OF THE ARROWHEAD REGION, NORTHEASTERN MINNESOTA, James K. Huber DESCRIPTION OF ROTASONIC CORE FROM THE TOIMI DRUMLIN FIELD AREA, Howard C. Hobbs
Guidebook 17. Field Trip Guidebook for Selected Areas in Precambrian Geology of Northeastern Minnesota
(Minnesota Geological Survey, 1987) Balaban, N.H., Editor
CONTENTS STRUCTURAL GEOLOGY OF THE BOUNDARY BETWEEN ARCHEAN TERRANES OF LOW-GRADE AND HIGH-GRADE ROCKS, NORTHERN MINNESOTA, P.J. Hudleston, R.L. Bauer, D.L. Southwick, D.D. Schultz-Ela, and M.E. Bidwell GEOLOGY OF THE KEWEENAWAN (UPPER PRECAMBRIAN) BEAVER BAY COMPLEX IN THE VICINITY OF SILVER BAY, MINNESOTA, James D. Miller, Jr. ROADLOG AND STOP DESCRIPTIONS FOR THE BEAVER BAY COMPLEX, James D. Miller, Jr., Paul W. Weiblen, and John C. Green
Guidebook 16. Field Trip Guidebook for Quaternary and Cretaceous Geology of West-Central Minnesota and Adjoining South Dakota
(Minnesota Geological Survey, 1987) Balaban, N.H., Editor
CONTENTS GEOMORPHOLOGY AND PLEISTOCENE GLACIAL GEOLOGY OF CENTRAL MINNESOTA, Barry Goldstein CRETACEOUS ROCKS ON THE EASTERN MARGIN OF THE WESTERN INTERIOR SEAWAY: A FIELD GUIDE FOR WESTERN MINNESOTA AND EASTERN SOUTH DAKOTA, George W. Shurr, Jay P. Gilbertson, Richard H. Hammond, Dale R. Setterholm, and Peter M. Whelan
Guidebook 15. Field Trip Guidebook for the Upper Mississippi Valley Minnesota, Iowa, and Wisconsin
(Minnesota Geological Survey, 1987) Balaban, N.H., Editor
CONTENTS KARST HYDROGEOLOGY OF SOUTHEASTERN MINNESOTA, E. Calvin Alexander, Jr. THE GEOLOGY OF THE ST. CROIX RIVER VALLEY, Mark E. Cavaleri, John H. MossIer, and Gerald F· Webers INTRODUCTION TO THE MIDDLE AND LATE ORDOVIClAN FIELD TRIPS, Robert E. Sloan THE MIDDLE ORDOVICIAN FOSSILS OF THE TWIN CITIES, MINNESOTA, Robert E. Sloan, William F. Rice, Eric Hedblom, and James M. Mazzullo THE MIDDLE AND LATE ORDOVICIAN STRATA AND FOSSILS OF SOUTHEASTERN MINNESOTA, Robert E. Sloan and Dennis R. Kolata THE MIDDLE AND LATE ORDOVICIAN STRATA AND FOSSILS OF IOWA, Dennis R. Kolata and Robert E. Sloan THE ROCK ELM DISTURBANCE, PIERCE COUNTY, WISCONSIN, William S. Cordua QUATERNARY GEOLOGY OF SOUTHEASTERN MINNESOTA, Howard C. Hobbs
Guidebook 14. Field Trip Guidebook for the Precambrian Terrane of the Minnesota River Valley
(Minnesota Geological Survey, 1982) Weiblen, Paul W.
The gently rolling farmland of southwestern Minnesota is a deceptive blanket over a rich record of the evolution of the earth's crust. The effects of Pleistocene glaciation dominate the area, and the landscape is characterized by a wide variety of glacial deposits which cover most of the bedrock geology. However, many outcrops are a product of glacial erosion and all the Precambrian outcrops in the Minnesota River Valley (MRV) were exposed by steambed erosion in the Glacial River Warren. This river drained Glacial Lake Agassiz prior to the disappearance of the ice sheet which prevented northward drainage to Hudson Bay. As can be seen in Figure 2, the Precambrian rocks of Minnesota occupy a central position in the North American continent. This position is reflected in the present day drainage pattern of the continent. Despite the low topographic relief (600-2,000 ft), there are three major drainage divides in Minnesota. The gentle relief and low elevation are also deceptive indicators of the crustal thickness of the North American continent in the region, which is on the order of 40 km. Despite its thickness and antiquity, the crust in Minnesota has remained geologically active. This is attested to in part by the fact that the Mesozoic north-south hinge line of the sedimentary basins involved in the formation of the Rocky Mountains crossed Minnesota. During the Paleozoic the region was involved in a number of oscillations of epicontinental seas. The distribution of the thin veneer of Phanerozoic rocks (maximum thickness less than 600 m), which were produced in these two episodes of crustal evolution, is shown in Figure 3 and in the geologic map of Minnesota on the frontispiece of the guidebook The geologic record is briefly outlined on the back cover. The thick, "stable" crust in the region is currently involved in the isostatic rebound associated with the disappearance of the Pleistocene ice sheets over North America. The locations of 12 recorded earthquakes in Minnesota are shown on Figure 3. The distribution of the earthquakes appears to be related to Precambrian tectonic features (Mooney and Morey, 1981), and may be related to reactivation of old fault systems in the course of isostatic rebound (Dutch, 1981). The Precambrian rocks in Minnesota define five distinct geologic terranes. The geology of these terranes is briefly outlined on the last page of the guidebook, and further details may be found in Sims and Morey ( 1972) and in more recent papers listed in Morey and others (1981). The outcrops in the MRV are part of the oldest terrane (I). They contain a record of a broad spectrum of igneous and metamorphic rocks which span a time interval from at least 3,500 to 1,800 m.y. Terrane I extends into central Minnesota where it is separated from the greenstone-granite terrane (II) to the north by a complex tectonic zone.
Guidebook 13. Field Trip Guidebook for the Archean and Proterozoic Stratigraphy of the Great Lakes Area, United States and Canada
(Minnesota Geological Survey, 1979) Morey, G.B.
This guidebook is prepared for the field excursion accompanying the Fifth Meeting of the International Union of Geological Sciences Subcommission on Precambrian Stratigraphy, to be held September 4-19, 1979, in the Great Lakes area, United states and Canada. Separate segments of the guidebook were prepared by the various field trip leaders and assembled by G. B. Morey, editor. M. J. Frarey coordinated the separate contributions for the Canadian part of the field excursion. The main purpose of the field excursion is to examine classic Precambrian sequences in the Lake Huron and Lake Superior regions, as a background for discussions on possible subdivision of the Archean and Proterozoic. The field excursion will be followed by technical sessions held in the Duluth, Minnesota, area.
Guidebook 12. Field Trip Guidebook for the Precambrian Geology of East-Central Minnesota
(Minnesota Geological Survey, 1979) Morey, G.B.
The bedrock geology of east-central Minnesota --generally bounded by latitudes 45 ON. to 47° N. and longitudes 92° 15' W. to 95° W. --is particularly interesting because the area contains a wide variety of igneous, metamorphic, and sedimentary rocks which span the entire range of Precambrian time. Unfortunately much of the bedrock is not well exposed. Those rock units that do crop out tend to occur as clusters 2 or 3 acres large or as strings 1 or 2 kilometers long; and these outcrop areas are widely separated by vast expanses of Pleistocene and Holocene materials as much as 140 thick. Therefore aeromagnetic and gravity data, and to a lesser extent, water-well and exploration records acquired over the years by the Minnesota and U.S. Geological Surveys were used to prepare a preliminary and somewhat generalized bedrock geologic map of east-central Minnesota at a scale of 1:500,000 (Morey, 1978).
Guidebook 11. Field Trip Guidebook for the Keweenawan (Upper Precambrian) North Shore Volcanic Group, Minnesota
(Minnesota Geological Survey, 1979) Green, John C.
The Upper Precambrian plateau lavas of the Lake Superior region were produced in response to tensional rifting of North America roughly 12001120 x 106 years ago (White, 1972a). Geological and geophysical evidence shows that they consist of a group of about eight separate volcanic accumulations which partly overlap in time and space as the locus and activity of rifting changed along what is now the Midcontinent Gravity High (Green, 1977). Nearly all of these lava accumulations ("plateaus") contain preserved sections which range in thickness from 2.5 to 7 km and are made of hundreds of individual flows. They have subsided centrally during and/or after eruption. Basalt of various compositions predominates; the most common type is nonporphyritic olivine tholeiite with unusually high Al content and ophitic texture. Transitional to weakly alkaline basalt is also common. Quartz tholeiite, basaltic andesite and rhyolite are moderately abundant in most of the plateaus, and icelandite (intermediate quartz latite) is found in some.
Guidebook 10. Field Trip Guidebook for the Western Vermilion District, Northeastern Minnesota
(Minnesota Geological Survey, 1979) Southwick, D.L.; Ojakangas, R.W.
The Vermilion Granitic Complex consists of granitic and migmatitic rocks of Archean age (2,700 m.y.) that are the westward extension into Minnesota of the Quetico gneiss belt of Ontario. The complex is chiefly light grayish-pink biotite granite (following the rock classification of Streckeisen, 1973) that grades into migmatite with increasing content of schistose inclusions. The major grayish-pink granite and its genetically related grani te rich migmati te were named the Vermilion Grani te by Grout (1923), who was the first to study the rocks in detail (Grout, 1923; 192~b,1926). Because migmatites are so abundant within the area Grout mapped as granite, and because other rock types such as quartz diorite and trondhjemite are important locally, the term Vermilion Granite has been replaced formally by the more inclusive term Vermilion Granitic Complex. Where the grayish-pink biotite granite that is the dominant component of Grout's Vermilion Granite is homogeneous, it has been renamed the Lac La Croix Granite, and it is understood to be a subunit within the Vermilion Granitic Complex.
Guidebook 9. Field Trip Guidebook for Stratigraphy, Structure and Mineral Resources of East-Central Minnesota
(Minnesota Geological Survey, 1979) Morey, G.B.; Davidson, D.M. Jr
Early in the 20th century, east-central Minnesota became the source of appreciable quantities of iron and ferromanganese, and even earlier, the source of a variety of granite products (Morey, 1977). Because of the obvious economic importance of the commodities to the state, most of the geologic work in east-central Minnesota focused on the Cuyuna iron-mining district or on the St. Cloud area where there are numerous granite quarries. Less attention was given to the geology of other parts of east-central Minnesota and to the possible presence of other mineral resources. This was true mainly because a fairly ubiquitous mantle of Quaternary materials made it difficult, time consuming and expensive for a company to establish the basic geologic information necessary to a successful exploration program. However, recent geologic work (Morey, 1978) has led to the recognition of several geologic environments that are similar to mineral-producing districts elsewhere in the world (Morey, 1977). Although these studies have shown that a variety of mineral occurrences may exist, most attention to date has focused on environments that may contain uranium. This road log starts at the Minnesota-Wisconsin border along the st. Louis River near Fond du Lac, the westernmost suburb of Duluth, Minnesota, and terminates near Sturgeon Lake on U.S. Interstate Highway 35 some 50 miles southwest of Duluth. Note that the mileages in this road log are approximate.
Guidebook 8. Field Trip Guide Book for Hydrogeology of the Twin Cities Artesian Basin
(Minnesota Geological Survey, 1972) Winter, T.C.; Norvitch, R.F.
This field trip focuses on several of the more significant aspects of the hydrogeology of the Minneapolis-St. Paul (Twin Cities) area. Emphasis is placed on the principal bedrock aquifer and the role of glacial drift in the hydrology. Stops (fig. 1) include field examination of the Jordan-Prairie du Chien aquifer, a well-screen manufacturing plant and its research well field, views of surface expression of partly buried bedrock valleys, an artificial-recharge site where experiments were run on deep-well water injection into the Prairie du Chien Group (carbonate rock), and a site where ground water discharges as a large spring from the Prairie du Chien Group. Although much glacial geology is seen along the trip route, emphasis is placed on the drift-filled bedrock valleys. An understanding of the hydrogeology of the Twin Cities area is becoming more critical as urbanization progresses. The population of the metropolitan area is 1,874,000 (1970), an increase of 350,000 since 1960. Predictions of population for the year 2000 are in the neighborhood of 3,000,000. Accompanying the population increase are questions concerning the adequacy of water supply; the effect of surface and subsurface waste disposal, paving, sewering, and other land-use practices on the hydrologic system; and the reasons for lake-level declines and accelerated eutrophication. The Twin Cities supply themselves and 13 surrounding municipalities with water from the Mississippi River. Water from a large number of wells within the cities is used mainly for industry and air conditioning. In addition, most suburban communities use ground water to the extent that ground water now exceeds surface water as a source of supply. Total ground water use in 1970 in the metropolitan area was 194 mgd (million gallons per day); 48 mgd was for domestic purposes, 75 mgd for air conditioning (based on 137 days), 100 mgd for industrial and commercial purposes, and 38 mgd for irrigation (based on 90 days). According to Norvitch (unpublished data), the amount of ground water that can be developed in the metropolitan area on a sustained basis is about 845 mgd. Optimum development of the ground-water resources will be possible only through proper management based on a thorough understanding of the hydrologic system.
Guidebook 7. Field Trip Guide Book for Geomorphology and Quaternary Stratigraphy of Western Minnesota and Eastern South Dakota
(Minnesota Geological Survey, 1972) Matsch, C.L.; Tipton, Merlin; Steece, F.; Rutford, R.H.; Parham, W.E.
The land surface of eastern South Dakota and southwestern Minnesota is underlain mainly by sediments of Quaternary age, and most of the land forms themselves are the result of erosional and depositional events that were closely controlled by climatic fluctuations during the Pleistocene Epoch. As a result of climatic changes, glacier ice advanced and retreated across the region, leaving a complicated stratigraphy of glacier-derived sediments. So recently did the last glacier deteriorate that the present landscape still retains the forms impressed by that complicated process. Even though geologists have been studying the Quaternary sediments of the region for almost 100 years, their interpretations are still controversial. Fundamental questions still incompletely answered are (1) how many drift sheets are present?; (2) where do the drifts fit into the mid-continent Quaternary time scale?; and (3) what is the surface distribution of each of the major drift units? During the last two decades the availability of aerial photography and topographic maps has facilitated detailed surface mapping projects. Absolute age determinations have been helpful in establishing a chronology for events during the last 40,000 years. The recognition, definition and tracing of lithostratigraphic units, begun several years ago, have added new dimensions to Quaternary studies in the region.
Guidebook 6. Field Trip Guide Book for Precambrian Geology of Northwestern Cook County, Minnesota
(Minnesota Geological Survey, 1972) Weiblen, P.W.; Davidson, D.M. Jr
An exceptionally complete record of Precambrian history is recorded in the rocks exposed in Cook County, Minnesota. In northwestern Cook County, in the vicinity of the Gunflint Trail the Lower Precambrian is represented by a metavolcanic succession, which was intruded by the somewhat younger Saganaga Tonalite. These rocks are unconformably overlain by the Middle Precambrian Animikie Group, consisting of the Gunflint Iron Formation and the Rove Formation. In northeastern Cook County, a gently dipping angular unconformity separates Middle Precambrian and Upper Precambrian strata. There, a thin basal sandstone, the Puckwunge Formation, is overlain by volcanic rocks of the North Shore Group. The Logan intrusions and the Duluth Complex intrude and truncate Middle and Upper Precambrian rocks and comprise the major part of the Upper Precambrian section in northwestern Cook County. Although the geology of Cook County was summarized by Grout and others (1959), geologic mapping since 1962 has considerably revised the earlier geologic interpretation. Because much of this work is unpublished as yet, a comprehensive summary is presented here. The discussion is meant to provide a framework for the specific aspects of the geology which the chosen stops illustrate.Mileages for this trip are listed by stop as distances in miles along Minnesota 12 (The Gunflint Trail) going both northwest from Grand Marais and southeast from Trails End Campground, a round-trip distance of about 120 miles. Figure 1 indicates the location of the Gunflint Trail as well as the general geology of the area. A larger scale geologic map of the field trip area together with all the field trip stops is shown in Figures 2 and 3, while the cross section on Figure 2 and the block diagrams of Figure 4 represent the gross structural relationships between the units encountered on the field trip.
Guidebook 5. Field Trip Guide Book for Precambrian Migmatitic Terrane of the Minnesota River Valley
(Minnesota Geological Survey, 1972) Grant, J.A.; Himmelberg, Glen R.; Goldich, S.S.
The Minnesota River Valley provides a tantalizing window onto the Canadian Shield on the eastern margin of the Great Plains, tantalizing because of the high grade of the metamorphism, and especially because of the antiquity of the rocks there exposed. Essentially, this is a migmatitic terrane of granitic gneisses with lesser amphibolitic gneisses, commonly with pyroxene, and biotite-rich gneisses, which may contain garnet, cordierite, sillimanite, anthophyllite, or hypersthene. Some of the rocks are greater than 3.0 b.y. in age, and they have been involved in metamorphism and deformation at least 2.6 b.y. ago. These events left rocks with a metamorphic grade in the upper amphibolite or granulite facies, and with a major structure that is similar throughout most of the exposed area. Later minor intrusions, dominantly mafic, cut the older rocks, and conglomerate and quartzite of the Sioux Formation of Late Precambrian age locally overlie them. Deep weathering of the gneisses formed a regolith about 100 feet thick, a part of which was reworked in the formation of Cretaceous deposits of sand and clay. Over this came the glacial deposits of the Pleistocene. With the formation of Lake Agassiz, drainage via Glacial River Warren scoured out the precursor of the present valley leaving an underfit present-day Minnesota River and the glimpse of the Precambrian described in the following pages. The granitic gneisses in the vicinities of Morton, Granite Falls, and Montevideo are among the oldest known crustal rocks. Like very ancient rocks in other parts of the world the gneisses have had a complicated history, and metamorphic changes have masked their original characters and obscured their age. Conservatively the age may be given as 3200 or 3300 m. y. Goldich and others (1970) have attempted to probe the metamorphic history and concluded that the gneisses date back to 3550 m.y. ago. Similarly old, or older gneisses (3600 to 4000 m.y.) have been reported from the Godthaab district, West Greenland (Black and others, 1971). Field and more detailed geochronological and geochemical investigations are being continued, and the nature of this work is briefly indicated in following sections.
Guidebook 4. Field Trip Guide Book for Paleozoic and Mesozoic Rocks of Southeastern Minnesota
(Minnesota Geological Survey, 1972) Webers, G.F.; Austin, G.S.
The Paleozoic rocks of southeastern Minnesota (fig. 1) were deposited from a marine sea which occupied the Hollandale Embayment (fig. 2), a shallow depression that extended northward from the Ancestral Forest City Basin (Iowa Basin) onto the cratonic shelf and into Minnesota and Wisconsin in Early and Middle Paleozoic time. The rocks that now remain within the embayment in Minnesota are bordered to the east by nearshore-facies Paleozoic rocks on the Wisconsin Arch, to the northeast by Precambrian rocks that constitute the Wisconsin Dome, and to the north and west by nearshore-facies Paleozoic rocks lying near the margins of the Hollandale Embayment and the Precambrian rocks of the Transcontinental Arch. The embayment overlies older basins and horsts that are bounded by largescale Precambrian faults (Sims and Zietz, 1967). Many smaller Paleozoic basins, depositional barriers, and faults within the embayment probably have resulted from relatively minor recurrent movements along Precambrian faults during Paleozoic time (Craddock and others, 1963).
Guidebook 3. Field Trip Guide Book for Precambrian North Shore Volcanic Group Northeastern Minnesota
(Minnesota Geological Survey, 1972) Green, John C.
Detailed mapping of the 85th Minnesota shore of Lake Superior began with A. E. Sandberg's study (1938) of the section between Duluth and Two Harbors. Grout and Schwartz (1939) and Gehman (1957) studied the intrusions and flows in eastern Lake County; Grogan (1940) mapped the lakeshore between Two Harbors and Split Rock River; Schwartz (1949) studied the Duluth area; and Grout and others (1959) mapped most of Cook County. James Kilburg (1972) has recently mapped the wedge of lavas just west of Duluth. Most of the data reported in this account derive from studies by the writer who, starting in 1965, has mapped the shoreline between Silver Bay and Grand Portage, with considerable reconnaissance inland and to the southwest (Green, 1966; 1968a; 1968b; 1970). The report does, however, also lean considerably on Grout and others (1959) and, for the Duluth-Two Harbors area, on Sandberg (1938). The field studies have been supported by the Minnesota Geological Survey, and most of the laboratory studies have been .supported by the National Science Foundation (Grant No's GP-5865 and GA-134ll). Sincere gratitude for this support is extended to both agencies. The writer's ideas have benefited from discussions with many other geologists concerned with Keweenawan rocks, especially including Bill Bonnichsen, D. M. Davidson, Jr., H. Hubbard, G. B. Morey, W. C. Phinney, P. W. Weiblen, and W. S. White. Trip will leave Duluth and head up-section in the southwestern limb of the basin.